1. Field of the Invention
The present invention relates to a method for manufacturing a pixel structure of a liquid crystal display (LCD). More particularly, the present invention relates to a method that uses only four masks to manufacture the pixel structure of a low temperature poly-silicon thin-film transistor liquid crystal display (LTPS-TFTLCD).
2. Descriptions of the Related Art
With low power consumption, light weight, low radiation and good portability, the LCD has become the dominant product in today's display market. Among various LCDs, the thin-film transistor liquid crystal display (TFT-LCD) technology falls into two main categories: amorphous silicon (referred to α-Si in short) and poly-silicon (Poly-Si). In the Poly-Si LCD, the low temperature poly-silicon (LTPS) is the new generation of manufacturing technology. As compared to the conventional α-Si LCD, the LTPS transistor has a carrier mobility of more than 200 times higher, and the display made therefrom features higher response, higher luminance, higher resolution and higher color saturation, thus, presenting a better picture quality. Moreover, with a lighter and slimmer profile, the LTPS display may have smaller components, resulting in shrinkage of more than 50% in area of the overall TFT assembly, thus, effectively reducing the power consumption and the manufacturing cost thereof. Therefore, the LTPS LCD is becoming increasingly popular on the LCD market.
FIG. 1A illustrates the schematic view of a pixel array formed on a substrate in a conventional LCD panel. The pixel array 1 comprises a plurality of scan lines 10 and data lines 11 intersecting with each other to define a plurality of pixel areas. Each of the pixel areas has a display unit 121 and a control element 123 formed thereon respectively. In addition, each of the scan lines 10 and the data lines 11 is connected to a plurality of pads 101, 111 disposed at the edges of the substrate respectively to transmit signals.
FIG. 1B illustrates a schematic cross-sectional view of such a structure, in which each of the pixel areas formed on a substrate 13 may be divided into a control area 131, a capacitance area 133 and a display area 135. In addition, a pad area 137 extending at the periphery of the substrate 13 is further illustrated in FIG. 1B.
In the conventional LTPS manufacturing technology of a pure PMOS product, seven masks are typically needed to perform exposure processes to form such a structure progressively. In particular, a poly-silicon layer 14 is firstly formed on the control area 131 and the capacitance area 133 by using a first mask. Next, a partial area of the poly-silicon layer 14 is doped with P+ ions by using a second mask to form a conductive structure 141. After the dielectric layer 15 is formed in the large area, a gate electrode layer 16 is formed on the control area 131, the capacitance area 133 and the pad area 137 respectively by using a third mask. Subsequently, once an intermediate dielectric layer 17 is formed to overlay the above-mentioned structure, an etch process is performed by using a fourth mask to form via holes to partially expose the poly-silicon layer 14 doped with P+ ions in the control area 131 and the capacitance area 133, and also to expose the gate electrode layer 16 in the pad area 137. Then, by using a fifth mask, a metallic conductive layer 18 is formed, which is electrically connected with the poly-silicon layer 14 through the via holes in the control area 131 and the capacitance area 133 respectively, and the gate electrode layer 16 in the pad area 137. Afterwards, a planarization layer 19 is formed, and a sixth mask is used to partially expose the metallic conductive layer 18. Finally, a transparent electrode 191 is formed by using a seventh mask, and is electrically connected with the metallic conductive layer 18.
Unfortunately, because masks are relatively expensive, the more masks that are used, the higher the manufacturing costs. Further, a manufacturing process involving masks is rather complex, which tends to decrease the yield rate of the products and therefore cannot satisfy the manufacturing requirements of current LCD. Moreover, because the display area 135 in a conventional structure still incorporates an ineffective multilayer structure, light from the substrate 13 has to transmit therethrough before it can result in a display. Even when such a structure is made of a transparent material, an adverse impact is still imposed on the transmittance of the display area 135, thus compromising the competitiveness of the products.
In view of this, it is highly desirable in the art to provide a method for manufacturing a low temperature poly-silicon liquid crystal display (LTPS-LCD) which uses fewer masks and can improve the transmittance of the display area.